This invention relates to sampled-data circuits and, more particularly, it relates to a switched-capacitor interpolation filter.
In a switched-capacitor filter of the type used in sampled-data circuits, the output signal is sampled and held at the clocking frequency. The frequency spectrum of the output signal ordinarily contains some extraneous components around the multiples of the sampling frequency. To get a smooth output, these components should be rejected. Heretofore, in order to eliminate the aforesaid undesired signal components, an off-chip analog smoothing filter was used in conjunction with a switched capacitor filter. However, this expedient added considerable complexity and size to the overall circuit as well as introducing other disadvantages. Another approach to the problem was to substantially increase the sampling rate for the main filter; but this resulted in a relatively high capacitor ratio spread and thus, a much larger silicon area for the filter, when the filter is implemented in integrated circuit form.
It is therefore one object of the present invention to provide a switched-capacitor interpolation filter that solves the problem of undesired frequency components without the disadvantages of prior expedients.
Another object of the invention is to provide a switched-capacitor interpolation filter capable of producing an output signal which is sampled and held at twice the clocking frequency of the filter while maintaining the same capacitor ratio spread of a filter clocked at half the sampling rate.